In industry, the demand for heat within a temperature range of 60° C. to 150° C. corresponds, among other things, to heating an external fluid (liquid or gas) with a large temperature differential between an initial temperature and a target temperature. The source fluids used as heat sources, from which heat is extracted, are usually liquid or gaseous effluents. The temperature differential may vary with the flow rate of these effluents.
The invention applies in particular:                to the heating of liquids such as water, wash water or other process fluids, in heating systems or other systems,        to the heating of gas for industrial drying, heating of rooms, or other uses.        
In particular, without the invention being limited to such an application, drying applications are widespread and are used in many fields: paper, food processing, waste water treatment, textiles, wood, paint, etc. The distribution of energy consumption associated with drying in France is as follows: 39% for the paper/cardboard industry, 23% for the agribusiness sector, 13% for chemistry, 11% for the materials sector, 2% in metallurgy, 2% in textile, and 10% for other industries. There are estimated to be just over 13,000 industrial drying facilities in France, which use a wide range of technologies. Convection drying is the most common method used industrially. The method consists of circulating a stream of gas that is as hot and dry as possible, over a material to be dried. This gas stream, usually air, provides the heat required to evaporate the liquid contained in the material, and carries away the (water) steam formed. The gas cools and gains humidity between its entering and exiting the drying facility, while the progressively dried material grows warmer.
One solution for achieving a large temperature differential when heating with an external fluid is to collect heat from a source fluid, which in the case of drying is the humid air generated by the drying facility.
To do this, it is known to use heat exchangers, such as tubular heat exchangers, finned heat exchangers with intermediate fluid, heat pipe exchangers, plate heat exchangers, or spiral heat exchangers. Specifications provided by the exchanger manufacturers indicate a thermal efficiency of between 40% and 90%. However, this ratio does not mean that the energy available in the hot (and humid) effluent will be recovered at such a yield. In the case of drying, for example, a simple exchanger between the extracted humid air and the incoming dry air only recovers a small amount of the thermal energy extracted from the humid air to increase the temperature of the incoming dry air. For high drying temperatures, it represents less than 8% of the energy introduced into the drying facility.
To contribute to the reduction of energy usage and CO2 emissions, the development of heat pumps (HP) is an attractive technological option for heating an external fluid.
A heat pump typically comprises a first heat exchanger, forming the evaporator, whose outlet is connected to the inlet of a second heat exchanger, forming the condenser, with a compression unit in between. The condenser outlet is connected to the evaporator inlet by means of an expansion unit. A coolant can thus flow between the evaporator and condenser to collect heat from the source fluid at the evaporator, and transfer heat to the external fluid at the condenser. In the case of drying, the extracted air can be cooled through the heat pump's evaporator (with condensation of the moisture), and the incoming air heated through the condenser to bring it to the desired temperature.
Drying facilities on record in France (for drying wood and sludge) confirm that heat pumps can reduce energy consumption.
However, conventional heat pumps can only achieve target temperatures limited to 60° C. In addition, the conventional thermodynamic cycles implemented in such heat pumps cannot reach very high condensation temperatures without crippling heat pump performance. The performance of existing heat pumps is therefore limited in the case of heating with large temperature differentials, and realistically they are not cost-effective.
To increase the performance of heat pumps for applications with large temperature differentials, particularly for the production of hot water in the residential and small business sectors, it is known to use CO2 heat pumps with a transcritical cycle. This solution is effective up to 90° C. due to the very high associated pressures.
Heat pumps using a hydrofluorocarbon (HFC) coolant in a transcritical cycle are described in the article “A Thermodynamic analysis of a transcritical cycle with refrigerant mixture R32/R290 for a small heat pump water heater,” Yu et al, Vol. 42, No. 12, p. 2431-2436, Dec. 1, 2010, and in document DE 103 27 953. Similarly to CO2 heat pumps with a transcritical cycle, these heat pumps are used for domestic applications such as heating water, for temperatures not exceeding 90° C. However, such heat pumps are not suitable for industrial applications where the temperatures to be reached are very high, in particular above 90° C., more particularly above 100° C., preferably above 120° C., and for example up to 150° C.
It is also known, for example from document DE 10 2008 047 753, to use heat pumps in a cascade arrangement. This arrangement provides an overall increase in performance due to the reduced temperature differential seen by each heat pump and to the possibility of adapting the coolant to the conditions of each heat pump. The more discrete the system the better the performance. In practice, however, criteria related to economic profitability limit the system to two heat pumps.
There is therefore a need for equipment for high temperature drying applications, and more generally high temperature heating applications with large temperature differentials, providing satisfactory performance and economic viability.